1. Field of the Invention
The invention generally relates to dynamic thermal-mechanical testing systems, and more particularly to apparatus for use therein that provides both self-resistive and self-inductive heating whenever a sufficiently large alternating electrical current is passed therethrough, as well as to a self-resistively and self-inductively heated specimen to be tested therewith.
2. Description of the Prior Art
Metallic materials play an indispensable role as an essential component of an enormous number of different products. One crucial property of such materials is their ability to conduct electricity. Absent operation at superconductive temperatures, a metallic object possesses a resistance to electrical current flow based upon its cross-sectional size, length and resistivity. Owing to this resistance, the object will generate heat whenever an electric current is passed therethrough. This form of heating is the so-called "self-resistive heating". Self-resistive heating finds use in a wide number of diverse applications.
Different materials, including those that are metallic, possess widely varying mechanical, metallurgical and other properties. As such, the specific properties required of a material for use in a given application are first determined followed by selection of a specific material that exhibits appropriate minimum values of these properties. An essential step in selecting a specific material is first to determine its properties of interest by testing specimens of each such material being considered.
Materials are tested in a wide variety of different ways. One such way, which is experiencing substantially increasing use, is dynamic thermal-mechanical testing. Here, a specimen is gripped at each of its two ends in a jaw system. The specimen is typically in the form of a small cylinder or sheet section of a given material and has a substantially uniform circular, rectangular or square cross-sectional area. An electric current is serially passed from one jaw assembly to another and through the specimen to generate a rapid, but controlled, heating rate throughout the specimen. Simultaneously therewith, various measurements are made of the specimen. Depending upon the specific measurements being made, the specimen either may or may not undergo controlled deformation while it is being heated. If the specimen is to be deformed, then this deformation can be accomplished by moving one of the two jaw assemblies, at a controlled rate with respect to the other, in order to impart either a controlled tensile or compressive force to the specimen. Alternatively, the specimen can be controllably struck by one of the jaw assemblies in order to impart a forging force thereto while current is passing through the specimen to controllably heat it. Physical measurements, such as illustratively specimen dilation and temperature, are typically made while heating and deformation are simultaneously occurring. This testing not only reveals various static properties of the specimen material itself, such as its continuous heating transformation curve, but also various dynamic properties, such as illustratively hot stress vs. strain rates and hot ductility; the dynamic properties being particularly useful in quantifying the behavior of the material that will likely occur during rolling, forging, extrusion or other material forming and/or joining operations. One system that provides excellent dynamic thermal-mechanical testing is the GLEEBLE 1500 system manufactured by the Duffers Scientific, Inc. of Poestenkill, N.Y. (which also owns the registered trademark "GLEEBLE" and is the present assignee). This system advantageously heats the specimen self-resistively in order to generate transverse isothermal planes throughout the entire specimen. Specifically, since each specimen generally has a substantially uniform transverse cross-section throughout its length, then the current density will be uniform throughout the entire specimen which will cause uniform heating over the entire specimen cross-section.
The specimens used in dynamic thermal-mechanical testing usually fall within a fairly wide range of sizes. Tensile specimens are often cylindrical in shape and may be on the order of approximately 6 to 12 millimeters in diameter and approximately 10 to 20 centimeters in length. Tensile specimens having a rectangular cross-section are also used from time to time. Compression specimens also tend to be cylindrical in shape and may be on the order of approximately 8 to 15 millimeters in diameter and also approximately 10 to 20 millimeters in length. For simulating strip annealing, a suitable specimen will have a rectangular cross-section and may be on the order of approximately 1 millimeter thick and approximately 170 millimeters wide and approximately 300 millimeters long.
Regardless of the specific specimen that is to undergo dynamic thermal-mechanical testing, relatively high currents generally must be passed through the specimen in order to produce the requisite level of self-resistive heating therein. The amount of electrical current that is required to heat a specimen to a given temperature and/or at a given heating rate generally depends upon a number of factors, for example: the specific heat of the material; its resistivity; the geometric shape of the specimen, such as its cross-sectional area and length; heat loss from the specimen to its surroundings, principally including but not limited to the jaw system used to grip the specimen; and the value of the final temperature to be attained. In practice and owing to the low resistances of most specimens, generally only a few volts or less need to be applied across the specimen to conduct the required current therethrough.
As noted above, a specimen is securely held between two jaw assemblies within a dynamic thermal-mechanical testing system and specifically by a grip contained within each such assembly. A series path is established to route heating current from one jaw assembly through the specimen to the other jaw assembly. For several reasons, the grips and jaw assemblies must be substantially larger in size than the specimen itself. First, electrical connections must be made to opposite ends of the specimen to conduct the current required to heat the specimen but without causing an appreciable voltage drop across each jaw assembly. As such, the jaw assemblies must be sufficiently large to provide a very low resistance path for high levels of current flowing therethrough. Second, to prevent the jaw assemblies, particularly if they are not water-cooled, from adversely melting or softening during high temperature testing, the jaw assemblies must provide a sufficient mass so that for a given current level, the jaw assemblies will remain appreciably cooler than the specimen and will heat at a significantly lower rate. Third, mechanical loads, as noted above in the form of tensile, compressive or forging forces, are applied through the jaw assemblies to deform the specimen while it is heating. As such, the jaw assemblies must be of sufficient size to safely transmit these forces to the specimen without experiencing any deformation themselves.
A number of dynamic thermal-mechanical tests require that essentially no longitudinal thermal gradients exist along the mid-span of the specimen. However, in practice, thermal gradients often occur between the ends of the mid-span during heating. The reason for this stems from the fact that the jaw assemblies, being of considerably greater thermal mass than the specimen, tend to conduct considerable quantities of heat away from the ends of the specimen while that specimen is being self-resistively heated. This, in turn, causes the opposing ends of the specimen to be significantly cooler than its mid-span.
Either one of two well-known techniques is often used to remedy this heat loss; however, each of these techniques possesses one or more drawbacks which disadvantageously limits its utility. First, both jaw assemblies themselves can be heated to the specimen temperature to prevent heat from being conducted from the specimen to these assemblies. Not only does this technique require the addition of supplementary heating equipment to heat the jaw assemblies but also necessitates, along with the added cost of this equipment, that a substantial amount of energy be consumed to heat these assemblies. In addition, if the jaw assemblies are heated to a sufficiently high temperature, these assemblies may become too ductile and will themselves deform while mechanical forces are being applied therethrough to the specimen. Second, for a given current passing through the specimen, the temperature of each end of that specimen can be increased by appropriately reducing the cross-sectional area of the specimen material appearing at that end. While, this technique is very reproducible, it adversely limits the maximum rate at which the entire specimen can be heated. Specifically, this technique involves drilling a number of holes into each end of the specimen near its jaw contact area in order to reduce the amount of material present thereat. Inasmuch as the material is reduced at each end, the cross-sectional area of the specimen at that end is decreased which, in turn, locally increases the current density occurring thereat. For a given amount of current passing through the specimen, the increased current density in the ends locally increases the heating rate and the final temperature of each end. Unfortunately, to generate sufficient heat at the ends to adequately compensate for the heat being lost from the specimen to the grips, a sufficient amount of material must be removed from each end of the specimen which may adversely cause the heating rate thereat to rise too rapidly to the point where specimen material melts and burns off each end prior to the mid-span of the specimen attaining a desired final temperature. To prevent this effect, the self-resistive heating current must be appropriately reduced which, in turn, adversely reduces the rate at which the entire specimen can heat. For example, an adequate cross-sectional reduction may necessitate that the cross-sectional area of each end be reduced to approximately 1/4 of its original value. However, to avoid excessive end temperatures from occurring in the specimen, the current that will pass through the specimen will need to be appropriately reduced such that the maximum heating rate of the entire specimen is only 1/4 of the value that would otherwise be used if the specimen had a uniform cross-sectional area throughout. Unfortunately, limiting the heating rate in this fashion artificially limits the thermal behavior of the specimen that can be measured by the testing system. Furthermore, this technique also tends to decrease the mechanical strength of the specimen to the point where purely mechanical testing thereof (e.g. application of tensile, compressive or forging forces) may not be possible. Specifically, as material is removed from each end of the specimen, the mechanical strength of that end decreases below that of the mid-span which, during the application of an appropriate force is likely to cause the specimen to prematurely fail at its end(s).
Thus, a general need exists in the art for a technique for use in conjunction with, for example, a dynamic thermal-mechanical testing system that can compensate for conductive heat loss occurring from a specimen under test to the jaw assemblies without requiring the use of heated jaw assemblies and which does not appreciably limit the maximum rate at which the entire specimen can be heated or reduce the strength of a specimen to be used for purely mechanical testing.
In addition to metallic materials, ceramic and composite materials have begun to play an increasing role as essential components of an enormous number of different products. Ceramic and composite materials tend to have high strength, and many are capable of withstanding high temperature environments. However, typically these materials have poor ductility. As such, a ceramic or composite material can fracture when an extreme thermal gradient is established thereacross. For instance, if a thermal gradient is established across a ceramic or composite material specimen, such that one end of the specimen is maintained at a higher temperature than the other end of the specimen. The high temperature end of the specimen expands to an increased physical size when compared to the physical size of the other, i.e., cool, end of the specimen. As a result of this thermal gradient, the physical size of the specimen tends to transition from a larger, high temperature end to a smaller, low temperature end. This transition places a mechanical stress upon the specimen in the form of the larger end pulling apart the smaller end. An extreme thermal gradient can generate sufficient tensile forces to fracture the specimen. Additionally, ceramic and composite materials are susceptible to rapid changes in temperature known as thermal shock. As with extreme thermal gradients, thermal shock can fracture ceramic and composite specimens.
Presently, thermal testing of ceramic and composite materials is accomplished in conventional furnaces using induction or radiant heaters. Mechanical test apparatus, having jaws, which grip both ends of a specimen or anvils that compress both ends of a specimen, apply mechanical test forces while the furnace heats the specimen. To achieve a thermal gradient along the length of the specimen, multi-chamber furnaces are typically used. In such furnaces, a high temperature heating chamber heats one end of a specimen while a low temperature heating chamber heats another end. To accurately control the temperature in a mid-span region of the specimen, one or more intermediary heating chambers are positioned physically and thermally between the low and high temperature chambers. Consequently, the multi-chamber oven establishes a thermal gradient along the length of the specimen. However, multi-chamber furnaces tend to establish small, i.e., generally flat, thermal gradients that extend over relatively long specimen lengths. As such, multi-chamber furnaces are not capable of testing specimens in environments similar to those in which the materials operate, i.e., extreme thermal gradients.
Thus, a specified need exists in the art for a technique for use in conjunction with, for example, a dynamic thermal-mechanical testing system that can heat a ceramic or composite material specimen and which can generate steep thermal gradients along the length of the specimen.